The present invention pertains to the art of medical diagnostic imaging. It finds particular application in the calibration of tomographic scanners in conjunction with the diagnosis of osteoporosis and will be described with particular reference thereto. It is to be appreciated, however, that the invention may find further application in conjunction with other calibrations of tomographic scanners and other medical diagnostic equipment.
Osteoporosis is a disease of long duration which is characterized by a bone mineral loss. The trabecular bone, which has a turnover rate about eight times higher than cortical bone, has been shown to be a sensitive early indicator of bone mineral loss. One method of monitoring for bone mineral loss has been to examine the spine, which is approximately 65% trabecular bone with an x-ray tomographic scan. The x-ray absorption characteristics of the trabecular bone and the reconstructed image are indicative of bone mineral content or density of the trabecular bone. One of the problems which has been encountered with x-ray tomographic scans of the trabecular bone has been calibrating the CT numbers or gray scale of the resultant image with sufficient precision to measure the amount of mineral loss accurately or to detect small mineral losses.
One solution for calibrating the gray scale images is set forth in U.S. Pat. No. 4,233,507 issued Nov. 11, 1980 to Donald J. Volz. In the Volz technique, calibration samples or phantoms are mounted in the patient supporting surface such that the calibration samples are imaged concurrently with the selected portion of the spine. This enables the gray scale or Hounsfield numbers for an imaged area of the spine to be compared with the known Hounsfield numbers for the calibration samples. However, the Volz techique has inherent inaccuracies in the calibration attributable to alterations in the beam caused by body tissues surrounding the spine.
The accuracy of the mineral content is determined by the accuracy of the CT numbers for the selected scanner and the chemical composition of the imaged bone tissue. Because the density of fat is less than that of soft tissue, it reduces the measured CT number by about 12 Hounsfield units per 10% of fat by weight. This produces an erroneous decrease in the measured bone mineral concentration. Concentrations of fat in the vertebral body of up to 40% have been reported. To reduce the magnitude of the CT number alteration attributable to fat, dual energy CT scanning techniques have been used.
The image reconstruction algorithms, x-ray spatial intensity compensation filters, x-ray beam filters, and x-ray beam hardness and scatter correction methods differ among various tomographic scanners. Commonly, scanner performance has been optimized by the use of empirical corrections to the above parameters to provide a flat response to water or tissue equivalent phantoms. These empirical corrections, in many instances, have not been linear to retain quantitative reconstructions over the entire field of view. These common, non-linear quantitative corrections in many instances are not constant for all body dimensions and x-ray excitation conditions. In particular, the x-ray beam quality is different for rays emerging through the center and the periphery of large bodies as contrasted with small body sections. This beam quality difference results in variations of the CT numbers for identical bone mineral materials.
This variation or shift in the CT numbers of otherwise identical bone mineral materials attributable to the amount and composition of surrouding soft tissue is uncorrected in the Volz technique. The Volz bone mineral reference samples are disposed peripherally where they are irradiated by off-focal radiation which has not passed through the same volume of soft tissue as the radiation passing through the spine. The peripheral, off-focal radiation may also introduce additional errors attributable to less accurate corrections made by shaped x-ray filters and compensators to off-focal radiation.
These problems become particularly acute when multiple energies of radiations are used. In particular, the volume of surrounding soft tissue, has relatively little effect on high energy radiation; whereas, the surrounding tissue more significantly affects low energy radiation. Thus, the body fat calibration corrections become particularly complex when utilizing a dual energy tomographic reconstruction technique, as is conventionally used to correct for the effects of the presence of the unknown amount of fat that has replaced tissue in the regions of interest.
The present invention contemplates a new and improved calibration method and apparatus which overcomes the above referenced problems and others to calibrate the CT numbers in the region of the spine more accurately.